1. Field of the Invention
The present invention relates to a motor, and more particularly to a current vector controlled synchronous reluctance motor and control method thereof.
2. Description of Related Art
Permanent magnet synchronous motor (PMSM) is a high efficiency motor including a stator and a rotor. The stator has multiple exciting coils for receiving a three-phase current. The rotor synchronously spins according to a rotating magnetic field generated from the exciting coils while phase currents flow through the exciting coils. However, the stator and the rotor of the PMSM are usually made of rare earth elements. The rare earth elements are expensive materials and may result in environmental pollution. Hence, to manufacture a motor without the rare earth elements is a trend nowadays.
A switched-reluctance motor is a high efficiency motor without the rare earth elements. With reference to FIG. 15, the switched-reluctance motor has a stator 61 and a rotor 62. The stator 61 has a U-phase coil 611, a Ū-phase coil 612, a V-phase coil 613, a V-phase coil 614, a W-phase coil 615 and a W-phase coil 616. The coils 611-616 are concentration coils and respectively have a current input terminal and a current output terminal. The rotor 62 has four poles 620.
With reference to FIG. 15, the symbols “” and “” respectively indicate the directions of phase currents flowing through the coils 611-616. Taking a plane of FIG. 15 as a reference plane, the symbol “” stands for the phase current flowing into the reference plane and the symbol “” stands for the phase current flowing outward the reference plane.
As an example, the U-phase coil 611 is opposite to the Ū-phase coil 612. The current output terminal of the U-phase coil 611 is connected to the current input terminal of the Ū-phase coil 612. While a phase current is flowing through the U-phase coil 611 and the Ū-phase coil 612, the coils 611, 612 induce magnetic fields in an opposite direction. For example, a magnetic pole induced by the U-phase coil 611 and adjacent to the rotor 62 is N magnetic pole. Another magnetic pole induced by the Ū-phase coil 612 and adjacent to the rotor 62 is S magnetic pole. Closed magnetic field lines 70 are formed along the U-phase coil 611, the rotor 62, the Ū-phase coil 612 and the stator 61.
With reference to FIG. 16, the position relationship of the poles 620 of the rotor 62 and teeth 617, 618 of the stator 61 is disclosed. The inductance varies with a rotational angle θ, of the rotor 62. Conventionally, a minimal inductance (Lmin) occurs when the rotor 62 induces a maximal reluctance at a certain rotational angle θr. A maximal inductance (Lmax) occurs when the rotor 62 induces a minimal reluctance at another certain rotational angle θr. The relationship of the rotational angle θr of the rotor 62 and the inductance is linear as indicated by A and B on FIG. 16.
The switched-reluctance motor generates an electromagnetic torque T1 according to the relationship of the rotational angle θr and the inductance. The electromagnetic torque T1 can be expressed as an equation:
            T      1        =                            ⅆ                      W            m                                    ⅆ                      θ            r                              =                        [                                    i              k              2                        ⁢            L            ⁢                                                  ⁢                                          ∂                                                      L                    k                                    ⁡                                      (                                          θ                      r                                        )                                                                              ∂                                  θ                  r                                                              ]                =                              1            2                    ⁢                      i            k            2                    ⁢                                    ⅆ                                                L                  k                                ⁡                                  (                                      θ                    r                                    )                                                                    ⅆ                              θ                r                                                          ,      k    =    1    ,  2  ,      3    ⁢                  ⁢    …  
wherein k stands for a phase and
      ⅆ                  L        k            ⁡              (                  θ          r                )                  ⅆ          θ      r      is a phase inductance increasing ratio and can be expressed as:
            ⅆ                        L          k                ⁡                  (                      θ            r                    )                            ⅆ              θ        r              =      {                                        M            ,                                                                              θ                r                            ⁢                                                          ⁢              is              ⁢                                                          ⁢              in              ⁢                                                          ⁢              the              ⁢                                                          ⁢              interval              ⁢                                                          ⁢              A                        ;                                                                          -              M                        ,                                                                              θ                r                            ⁢                                                          ⁢              is              ⁢                                                          ⁢              in              ⁢                                                          ⁢              the              ⁢                                                          ⁢              interval              ⁢                                                          ⁢              B                        ;                                                            0            ,                                                              θ              r                        ⁢                                                  ⁢            is            ⁢                                                  ⁢            in            ⁢                                                  ⁢            the            ⁢                                                  ⁢            interval            ⁢                                                  ⁢                          C              .                                          
wherein M is a positive real constant.
However, the equations mentioned above are only adapted for a motor operating in a low speed. When the motor operates at a high speed or is connected to a load which is gradually increased, the phase inductance increasing ratio is not a constant anymore, resulting in high torque ripple and noise.